1. Field of the Invention:
The present invention relates to an energization controller for controlling ON/OFF of an H-type switching circuit for feeding chopped electricity to the coils of an electric motor.
2. Description of Related Art:
In general, an energization controller of this type controls the energization by detecting a value of current flowing through the coil by a resistor or the like, by comparing the detected current value with a predefined reference current value in a binary fashion, by turning on a switching element when the reference current value is greater than the detected current value and by turning off the switching element when the reference current value is equal to or falls below the detected current value. Thus the switching element repeats ON and OFF so that the average value of the current flowing through the coil is controlled to a value corresponding (or close) to the reference current value. By the way, in the energization controller of this type, the higher the ON/OFF frequency (chopping frequency) of the switching element, the more the energy loss increases in the switching element, thus increasing calorific power. Therefore, it is not preferable to increase the chopping frequency too much. When the chopping frequency is too low, in contrast, a difference between the reference current value and the controlled current value increases, thus dropping the accuracy of the control. Further, it is not preferable for the chopping frequency to fall within the human audible frequency range because mechanical vibrations caused by the chopping of the current sounds like noise to human ears.
There has been proposed in Japanese Patent Laid Open No. Hei. 8-172793 an energization controller which comprises enable signal generating means for generating an enable flag signal which is set at a first level every time a signal for turning on/off the switching element is switched from ON to OFF or when switched from OFF to ON and which is set at a second level every time a reference chopping timing which occurs almost at constant period comes, and which causes the chopping frequency to approach a desired frequency (in the vicinity of the upper limit of the human audible frequency range, e.g. 15 KHz) by prohibiting the energization of the coil from being switched from OFF to ON or from ON to OFF during the period when the enable flag signal is on the first level.
In this energization controller, an H-type switching circuit is used to control a motor energizing current by a PWM to smooth the rise and fall of the energization and to control switching modes to improve the shortage of torque.
For instance, as shown in FIGS. 13a and 13b, the H-type switching circuit comprises a first switching element 3a interposed between one end of an electric coil 1a of the electric motor and a first power line 2a , a second switching element 3b interposed between the other end of the electric coil 1a and a second power line 2b, a first diode D1 which is interposed between one end of the coil 1a and the second power line 2b and which permits a current to conduct from the latter to the former and a second diode D2 which is interposed between the other end of the coil 1a and the first power line 2a and which permits a current to conduct from the former to the latter.
A rotational driving current flows through the electric coil 1a as shown in FIG. 13a when the first and second switching elements 3a and 3b are both turned on and a feedback current to the power source caused by an induced voltage of the electric coil 1a flows through the electric coil 1a as shown in FIG. 13b when they are both turned off. A pulsating current whose falling speed is relatively quick flows through the electric coil 1a by repeatedly turning on and off the energization as described above alternately by the PWM control. This switching mode will be referred to as "hard-chopping" in the present specification. Energy generated by the electric coil 1a is supplied (regenerated) to the first power line 2a and the current sharply decreases at the time interval during which the first and second switching elements 3a and 3b are both turned off as shown in FIG. 13b in the hard-chopping mode.
A pulsation current whose falling speed is relatively slow flows through the electric coil 1a by alternately repeating states of turning on both of the first and second switching elements 3a and 3b as shown in FIG. 14a (similarly to FIG. 13a ) and of turning off only the first switching element 3a while keeping the second switching element 3b ON as shown in FIG. 14b. This switching mode will be referred to as "soft-chopping" in the present specification. The current decreases moderately during the period in which the first switching element 3a is OFF and the second switching element 3b is ON as shown in FIG. 14b in the soft-chopping mode.
The energization controller disclosed in the above-mentioned Japanese Patent Laid Open No. Hei. 8-172793 has eliminated the problem of shortage of rotation torque by selecting the above-mentioned "hard-chopping" and "soft-chopping" based on the number of revolutions of the motor and required torque.
A switched reluctance motor (hereinafter referred to as an "SR motor") comprises a rotor which is generally constructed so that pole portions protrude outward, a stator which is constructed so that pole portions protrude inward and a coil wound in concentration on each pole of the stator. In the motor, magnetic mechanical resistance varies depending on rotational position of the rotor with respect to the stator, so that inductance L of the coil varies accordingly. When this inductance L is small, the coil current falls with a desired falling speed in either chopping mode of the above-mentioned hard-chopping and soft-chopping modes, so that it is possible to approach the desired chopping frequency in the above-mentioned prior art energization controller. However, when the inductance L is large, the falling speed of the coil current slows down during the above-mentioned soft-chopping mode and the state of the reference current value lasts even when the reference chopping timing passes, so that switching of the signal for turning the on/off switching element from OFF to ON is delayed. Due to that, the chopping frequency drops and mechanical vibrations caused by the chopping sounds like noise to human ears.
Although the chopping frequency will not drop in the above-mentioned hard-chopping mode even when the inductance L is large, the amplitude of the pulsation of the current caused in switching the switching element ON/OFF becomes large, so that pulsation of magnetic attractive force applied to the rotor of the SR motor becomes large, thus causing vibrations and loud noise.
Accordingly, it is an object of the present invention to keep the desired chopping frequency without increasing the current amplitude in the energization controller of the electric motor.